Lateral photovoltaic device for near field use

ABSTRACT

A device, method and process of fabricating an interdigitated multicell thermo-photo-voltaic component that is particularly efficient for generating electrical energy from photons in the red and near-infrared spectrum received from a heat source in the near field. Where the absorbing region is germanium, the device is capable of generating electrical energy by absorbing photon energy in the greater than 0.67 electron volt range corresponding to radiation in the infrared and near-infrared spectrum. Use of germanium semiconductor material provides a good match for converting energy from a low temperature heat source. The side that is opposite the photon receiving side of the device includes metal interconnections and dielectric material which provide an excellent back surface reflector for recycling below band photons back to the emitter. Multiple cells may be fabricated and interconnected as a monolithic large scale array for improved performance.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.13/469,842 filed on May 11, 2012 which issued as U.S. Pat. No. 9,056,006on Jun. 23, 2015, the contents of which are incorporated by reference asthough fully set forth herein.

BACKGROUND

The present invention relates generally to thermophotovoltaic devicesfor generating electrical energy, and more particularly to the processof fabricating an interdigitated multicell device that is particularlyefficient for generating electrical energy from a heat source byoperation in the near field. In this and similar applications, a veryhigh efficiency device is required that absorbs more in band than out ofband photons. Inefficient thermophotovoltaic cells cause a drop in thetemperature of the emitter used to form near fields and this results ina poorer photon transmission to the thermophotovoltaic cell.

Photovoltaic energy conversion is a direct conversion process thattransfers electromagnetic energy in the form of photons from an emitterto a photovoltaic device for generation of electrical energy by thereceiving photovoltaic device. The most widely recognized photovoltaicdevices are based on semiconductor technology and optimized foroperation in the solar spectrum, i.e. light from the sun, while lesswell-known semiconductor photovoltaic devices that are optimized foroperation in the infrared and near-infrared spectrum are referred to asthermophotovoltaic devices. For operation in the solar spectrum,photovoltaic devices generate electrical energy by absorbing photons ofenergy in the 1.0 to 5.0 electron volt range. Thermophotovoltaic devicesgenerate electrical energy by additionally absorbing lower energyphotons in the 0.2-1.0 electron volt range. To provide the best matchfor a thermophotovoltaic device and the incoming spectrum of photonenergy from a low temperature heat source requires a narrow bandgap ofthe semiconductor material of the thermophotovoltaic device. Photonshaving energy greater than or equal to the semiconductor materialbandgap can generate electrical energy, while lower energy photonsgenerate heat and result in a loss of efficiency. In addition, photonenergy well in excess of the semiconductor material bandgap is alsopartially lost as heat.

Thermophotovoltaic devices comprise one or more semiconductor P-Njunctions or diodes that collect and separate the electron/hole pairsthat are generated by the absorption of photon energy and therebyproduce electrical energy. The characteristics of a thermophotovoltaicdevice within a thermophotovoltaic system provide opportunities foroptimizing the operation of that system through use of additionalcomponents such as concentrators, filters, reflectors and selectiveemitters. Multiple thermophotovoltaic devices may also fabricated on asingle substrate for large array applications.

Thermophotovoltaic devices receive photon energy from a relatively hotemitter separated from a relatively cool receiving thermophotovoltaicdevice by a gap. When the gap spacing between an emitter and receiver isone micron or less, which is considered to be near field operation,greater power transfer is achieved than that predicted by Planck's Lawfor black body radiation for far field operation. For gap spacing ofone-tenth micron, energy transfer increases by factors of five or moreare possible when compared with that predicted by far field theory.However, such narrow gap spacing generally requires a vacuum in a gapbetween a relatively hot emitter and a relatively coolthermophotovoltaic device to reduce the effects of heat conduction.Although increases in energy transfer between an emitter and athermophotovoltaic device receiver may be achieved by increasing thetemperature of the emitter, material limitations place a practical limiton a maximum temperature of operation of these devices. Also, at higherdevice temperatures, intrinsic carrier generation within thesemiconductor device prevents effective collection of electrons.

Although in theory any material that can support the temperature can beused as an emitter, certain advantages, such as a more favorable outputspectrum, may pertain to selective materials. Although not limited tothese examples, materials used for, emitters in thermophotovoltaicsystems include: single crystal silicon, polycrystalline silicon,silicon carbide, tungsten, rare-earth oxides and photonic crystals.Thermophotovoltaic devices may be fabricated from materials such assilicon, germanium, gallium antimonide, indium gallium arsenideantimonide, indium gallium arsenide, and indium phosphide arsenideantimonide.

Previous fabrication methods for producing thermophotovoltaic deviceshave employed thin active layers comprising multiple narrow cells on awindow-like substrate. These isolated cells are interconnected in seriesfor producing a higher voltage at lower output current levels in orderto minimize power loss. Increased production costs result from theadditional complexity of fabricating multiple cells on a commonsubstrate.

These previous devices also employ a collection method whereelectron-hole pairs created from incoming photons travel perpendicularto the plane of the device to reach their respective collecting regions.For purposes of distinction, we shall call this situation perpendicularcollection. Perpendicular collection methods are often the only optionthat can be employed in prior multicell devices because many of the lowbandgap materials require junction formation by epitaxy, as the commonmethods of diffusion and ion implantation have not proved successful todate. If epitaxy alone is used, this inherently gives a perpendicularconstruction. A low bandgap material is required to collectpredominantly low energy infrared and near-infrared photons emitted fromrelatively low temperature sources.

A perpendicular collection method also requires a lateral conductionlayer (LCL) for conduction of photocurrents from one cell to the nextcell of a multicell device. The use of perpendicular collection layermethods and lateral conduction layers requires trade-offs. A thick,heavily doped layer is desirable to minimize resistive losses in thelateral conduction layer but a lightly doped region is desired tominimize absorption of below-band photons, which can only be convertedinto heat.

The prior art photovoltaic devices have been designed for far fieldoperation and have focused on obtaining the maximum output from theimpinging spectrum to obtain an attractive overall system cost. Theinfluence of near field operation is now illustrated for the case wheregermanium is used as the collecting semiconductor material. The maximumoutput consideration requires a thick germanium layer of at least 150microns. However, such a thick layer also creates severe problems informing a multicell device which requires physical isolation between theunit cells making up the device.

Another factor promoting thick absorbing layers of germanium in priordevices is the fact that germanium has two band gaps; a direct bandgapat 0.80 eV and an indirect gap at 0.67 eV. Indirect gaps have smallerabsorption coefficients so that if one is trying to maximize photoncollection for both gaps a thick absorption layer is required.

Some of the prior art multicell structures also have unresolved issuessuch as ohmic contact caused by the doping concentration compromiserequired in the LCL. Minimizing contact resistance for these devicesrequires elaborate and complex processes, such as tunnel junctions,which make these cells less competitive cost wise for an energyconversion system.

The epitaxial only construction of prior art devices results in theuppermost region being uniformly heavily doped across the entire face ofthe device in order to make good ohmic contact where contacted bymetallization. This gives an excessive area of heavy doping whichresults in high below band absorption in this layer similar to the casewith the LCL.

The distinction “epitaxial only” is used because when all of the regionsare formed by epitaxy the collection configuration must beperpendicular. If at least one of the regions can be formed by diffusionor ion implant then a lateral collection construction may be possible ifthe diffusion length allows for reasonable geometries. Again, fordistinction, we define a lateral collection method as one where theminority carrier flow to the collecting regions is parallel to the planeof the photovoltaic device.

Also, the heavily doped upper layer interferes with the ability to makea high efficiency back surface reflector because heavily doped regionsdo not make the best reflectors. An efficient back surface reflector isa very important component in an efficient near field energy conversionsystem.

SUMMARY

The disclosed invention is a device, method and process of fabricatingthe device, the device being an interdigitated multicellthermophotovoltaic component that is particularly efficient forgenerating electrical energy from photons in the red and near-infraredspectrum received from a heat source in the near field. For theembodiment where the absorbing region is germanium, the device iscapable of generating electrical energy by absorbing photon energy inthe greater than 0.67 electron volt range corresponding to radiation inthe infrared and near-infrared spectrum. Use of germanium semiconductormaterial, which has bandgaps of 0.67 and 0.80 electron volts, provides agood match for converting energy from a low temperature heat source. Theside that is opposite the photon receiving side of the device includesmetal interconnections and dielectric material which provide anexcellent back surface reflector for recycling below band photons backto the emitter. The device is designed to receive photon energy from aradiating emitter spaced less than a micron from its surface. Multiplecells may be fabricated and interconnected as a monolithic large scalearray for improved performance.

Unlike prior devices that suffer from the limitations imposed byperpendicular collection methods and lateral conduction layers, thepresent device relies on a lateral collection method. The electron-holepairs that are generated travel laterally between N+ collectionjunctions and P+ ohmic contacts that are disposed side by side on thesurface of the collecting material. In the case of germanium, only asingle layer of lightly doped P-type material about ten microns thick isrequired for collection. The N+ and P+ regions may be formed byconventional diffusion or ion implantation. For germanium the heavilydoped regions can be minimized in area because the high lifetime allowsfor wide separation of the ohmic contacts. Isolation cuts do not requireadditional openings in the bottom of the trench to form metallizationcontact to an LCL and are therefore simpler to fabricate than in theperpendicular case. In forming the collection region which is adjacentto the undoped substrate, a thin initial layer of medium concentrationis used to create a reflecting layer for minority carriers. Thiseffectively isolates the collecting region from the substrateheterojunction which can be heavily damaged due to lattice mismatch.

The lateral collection method can only be employed with reasonably highlifetime or long diffusion length materials. The advantages of thedescribed invention include high efficiency of in band to below bandabsorption. Because the un-doped substrate, which may be galliumarsenide in the germanium embodiment, is a wide band gap material, itdoes not absorb the photons which are in band for the collecting region.The diffusion length in moderately doped germanium is much longer thanthe junction spacing, so the horizontal collection method remains veryefficient when this material is used. Also, the below band photonabsorption is minimized by the un-doped nature of the substrate. Thedoped portions of the collecting region are made very thin so that belowband photon absorption is minimized in these areas. Another advantage ofthe present approach is that a thin collecting layer is a verycost-effective method of fabricating the device. Full thicknesssubstrates of collecting materials such as germanium and galliumantimonide are generally very expensive. Germanium on Gallium Arsenidecan be grown on large substrates using inexpensive vapor depositionmethods as opposed to Molecular Beam Epitaxy (MBE) or Meal OrganicChemical Vapor Deposition (MOCVD) which are commonly used for many ofthe epitaxy only infrared sensors. Since there is no lateral conductionlayer, isolation trenches may be made narrow and refilled to facilitatemetallization. There is no absorption that would otherwise take place ina lateral conduction layer, thus minimizing below band absorption.

The invention uses a thin layer of collecting material that isapproximately 10 microns thick that, in the case of germanium, providesonly a minimal cross section for below band or free carrier absorption.It takes advantage of a nonlinear variation in the absorptioncoefficient versus wavelength that makes the magnitude of collectionfrom a low temperature source practical when combined with near fieldenhancement. Although some loss of response may occur due to the thinlayer of germanium, operation in the near field multiplies thephotocurrent so that an attractive power density results that is aboutan order of magnitude greater than typical solar cells.

An embodiment of the present invention is a multicell thermophotovoltaicdevice for generating electrical energy, which comprises: an undopedsubstrate common to all cells of the multicell thermophotovoltaicdevice, the substrate having a first surface for receiving infrared andnear-infrared photon energy from an emitter heat source in a near fieldand a second surface opposite the first surface forming an interfacewith a plurality of thin collection region layers; where the pluralityof thin collection region layers for generating electron-hole pairs fromabsorbed photon energy from the substrate; N+ collection junctions andP+ ohmic contact regions formed in a surface of each of the plurality ofthin collection region layers opposite the substrate, the N+ collectionjunctions for collecting electrons and the P+ ohmic contact regions forcollecting holes from the electron-hole pairs traveling laterally in theplurality of collection region layers; isolation cuts between individualcells of the multicell thermovoltaic device, the individual cells formedby isolating adjacent P+ ohmic contact regions and N+ collectionjunctions within each of the plurality of collection region layers;dielectric layers formed on the surface of each of the plurality ofcollection region layers opposite the substrate and on surfaces of theisolation cuts; and metal interconnections and vias in the dielectricmaterial for interconnecting N+ collection junctions and P+ ohmiccontact regions within and between individual cells in an array forforming the multicell thermophotovoltaic device.

Another embodiment of the present invention is a method for generatingelectrical energy using a multicell thermophotovoltaic device, whichcomprise the steps of: positioning a first surface of an undopedsubstrate common to all cells of the multicell thermophotovoltaic devicefor receiving infrared and near-infrared photon energy from an emitterheat source in a near field and forming an interface with a plurality ofthin collection region layers on a second surface of the substrateopposite the first surface of the substrate; generating electron-holepairs from absorbed photon energy from the substrate by the plurality ofthin collection region layers; forming N+ collection junctions and P+ohmic contact regions in a surface of each of the plurality of thincollection region layers opposite the substrate, the N+ collectionjunctions for collecting electrons and the P+ ohmic contact regions forcollecting holes from the electron-hole pairs traveling laterally in theplurality of collection region layers; providing isolation cuts betweenindividual cells of the multicell thermovoltaic device, the individualcells formed by isolating adjacent P+ ohmic contact regions and N+collection junctions within each of the plurality of collection regionlayers; forming dielectric layers on the surface of each of theplurality of collection region layers opposite the substrate and onsurfaces of the isolation cuts; and interconnecting N+ collectionjunctions and P+ ohmic contact regions using metal interconnections andvias in the dielectric material within and between individual cells inan array for forming the multicell thermophotovoltaic device.

Yet another embodiment of the present invention is process forfabricating a multicell thermophotovoltaic device which comprises thesteps of: epitaxially growing a thin layer of lightly doped P-typegermanium on a substrate of undoped gallium arsenide; forming P+ regionsand N+ regions in the lightly doped P-type germanium layer opposite thesubstrate by ion implantation; providing a lateral collection region inthe lightly doped P-type germanium layer between the P+ region and theN+ region for electron-hole pair generation by absorbed incidentphotons; etching isolation cuts through the P-type germanium layer downto the substrate for isolating diodes formed by the P+ region and N+region and the P-type germanium layer; depositing a dielectric layerover the surface comprising the P+ region, the N+ region, the P-typelayer and the isolation cuts; forming vias to expose contact areas tothe P+ regions and the N+ regions; and forming ohmic contacts bymetallization patterns for interconnecting diodes and cells.

Note that reference to perpendicular and lateral collection regions usedherein are for purposes of distinction with the prior art. The termsrefer to the direction of flow of minority carriers from the point ofgeneration to the respective ohmic contact collecting areas.Perpendicular motion is perpendicular to the plane of the device whichis generally a flat substrate. Lateral motion is parallel to the planeof the substrate.

Note also that the terms N and P used throughout this description may bereversed if the opposite polarity semiconductor doping is used, i.e.,N-type collector instead of the P-type collector illustrated.

In some cases, an ohmic contact may be formed by growth of an epitaxiallayer which is subsequently etched in the form of the contacting regioninstead of a diffusion or implant. The complimentary ohmic contact mayalso be formed by a second epitaxial growth or preferably a diffusion orimplant. The key distinction with prior art is that these regions aredisposed laterally and not in a perpendicular fashion.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects and advantages of the presentinvention will become better understood with regard to the followingdescription, appended claims, and accompanying drawings wherein:

FIG. 1A depicts a cross-section view of a single diode prior art deviceemploying a perpendicular collection method;

FIG. 1B depicts a symbolic electrical representation of the device shownin FIG. 1A;

FIG. 2A depicts a cross-section view of multiple diode cells of a priorart device employing a perpendicular collection method;

FIG. 2B depicts symbolic electrical representation of the device shownin FIG. 2A;

FIG. 3A depicts a cross section view of a single diode device employinga lateral collection method;

FIG. 3B depicts a symbolic electrical representation of the device shownin FIG. 3A;

FIG. 4A depicts a cross section view of a multiple diode cell deviceemploying a lateral collection method;

FIG. 4B depicts symbolic electrical representation of the device shownin FIG. 4A;

FIG. 5A depicts a perspective view of an array of multiple diode cellsformed by creating alternating interdigitated N+ collection junctionsand P+ ohmic regions in a P-type collecting region and employing alateral collection method according to the present invention;

FIG. 5B depicts a symbolic electrical representations of the deviceconfiguration shown in FIG. 5A;

FIG. 5C illustrates a longitudinal sectional view taken at section A-A′depicted in FIG. 5A;

FIG. 5D illustrates a lateral sectional view taken at section B-B′depicted in FIG. 5A;

FIG. 6A through FIG. 6F depict typical steps of a fabrication processfor forming a multicell device shown in FIG. 4A according to the presentinvention;

FIG. 6A depicts a thin layer of lightly doped P type germaniumepitaxially grown on a substrate of undoped gallium arsenide;

FIG. 6B depicts P+ regions and N+ regions formed in the P− layer by ionimplantation;

FIG. 6C depicts trenches cuts through the P type germanium layer down tothe substrate for providing isolation between the diodes formed by theP+ and N+ regions and the P type germanium layer;

FIG. 6D depicts a dielectric layer deposited over the surface of thedevice;

FIG. 6E depicts vias that are cut to expose contact areas to the P+ andN+ regions of the device; and

FIG. 6F depicts ohmic contacts formed by metallization patterns forinterconnection purposes.

DETAILED DESCRIPTION OF THE DRAWINGS

Turning to FIG. 1A, FIG. 1A depicts a cross-section view 110 of a singlediode prior art device 100 fabricated by growing epitaxial layers of P+material 116, N+ material 118, and N-type material 120 on asemi-insulating substrate 122 and employing a perpendicular collectionmethod. FIG. 1B depicts a symbolic electrical representation 110 of thedevice shown in FIG. 1A that comprises a single thermophotovoltaicdiode. The semi-insulating substrate 122 is un-doped so that it acts asan insulator but has the requisite lattice spacing to support epitaxialgrowth processes. Alternate layers of an N+ region 118, N− region 120and P+ region 116 are epitaxially grown on the substrate 122. Access tothe N+ region 118 is obtained by etching a first cut 126 through the P+region 116 and the N region 120 to the N+ region 118. To preventunwanted electrical contact a dielectric layer 112 is deposited on theexposed surface of the cut. A second cut 124 down to the semi-insulatingsubstrate 122 isolates N+ region 118. Ohmic contact regions to the diodeare made by forming openings or vias in layer 112 to allow an anodeohmic contact 114 by a deposited metal film to the P+ region 116, and aan opening to allow a cathode ohmic contact 134 by a deposited metalfilm to the N+ region 118.

When photons 170 illuminate the N region 120, hole-electron pairs aregenerated in the N region 120 and the holes migrate to the P+ region 116and the electrons migrate to the N+ region 118. This results in athermophotovoltaic potential and current can flow externally between theanode ohmic contact 114 and the cathode ohmic contact 134. Note that thehole-electron migration is in a direction perpendicular to the plane ofthe device.

Turning to FIG. 2A, FIG. 2A depicts a cross-section view of multiplediode cells of a prior art device 200 fabricated by growing diode layersof P+ material 216, 236, 256, N+ material 218, 238, 258 and N-typematerial 220, 240, 260 on a semi-insulating substrate 222 employing aperpendicular collection method. FIG. 2B depicts symbolic electricalrepresentations 210, 230, 250 of the device shown in FIG. 2A. Note thatFIG. 2A represents a series connection of diodes 210. 230, 250. Thesemi-insulating substrate 222 is doped so lightly that it acts as aninsulator but has the requisite lattice spacing to support epitaxialgrowth. The N+ regions 218, 238, 258 and the P+ regions 216, 236, 256are isolated from each other by etching a first cut 226, 246, 266through the P+ regions 216, 236, 256 and the N regions 220, 240, 260 tothe N+ regions 218, 238, 258. A second cut 224, 244, 264 is etched downto the semi-insulating substrate 222 to isolate the diode cells.Isolation in the cuts is provided by dielectric layer 212 that hasopenings or vias to allow an anode ohmic contact by a deposited metalfilm 214 to the P+ regions 216, 236, and 256, and openings or vias 226,246, and 266 to allow a cathode ohmic contact by the same depositedmetal film. The resultant structure is a string of diode cells isolatedfrom each other on the wafer and connected in a series string.

When photons 270 illuminate the N regions 220, 240, 260, hole-electronpairs are generated in the N regions 220, 240, 260 and the holes migrateto the P+ regions 216, 236, 256 and the electrons migrate to the N+regions 218, 238, 258. This results in a thermophotovoltaic potentialand current can flow externally between the anode ohmic contact 214 andthe cathode ohmic contact 274 at opposite ends of the diode string. Notethat the hole-electron migration is in a perpendicular direction.

Turning to FIG. 3A, FIG. 3A depicts a cross section view of a singlediode device 300 fabricated by forming an N+ collection junction 318 anda P+ ohmic contact region 316 in a lightly doped P-type layer 320 andemploying a lateral collection method. FIG. 3B depicts a symbolicelectrical representation 310 of the device shown in FIG. 3A thatcomprises a single photovoltaic diode. Photon collection takes place inthe lightly doped P-type layer 320 that has been grown on a substrate322 of un-doped wide band gap material such as semi-insulating galliumarsenide or silicon. The collection layer 320 may be either N-type orP-type depending on the material selected and the other regions dopedaccordingly. The N+ region 318 and the P+ region 316 may be formed byany appropriate technique such as diffusion, ion implantation, oretching of a grown layer. The surface is protected by dielectric layer312 that has openings or vias to allow an anode ohmic contact 314 by adeposited metal film to the P+ region 316, and a an opening to allow acathode ohmic contact 334 by a deposited metal film to the N+ region318. In many cases the metal layer is common to all contacts. Photoncollection occurs in the P-type layer 320 which may be doped somewhathigher in the vicinity of the substrate 322 than the remainder of the P+region 320 to form a diffusion barrier to keep minority carriers frommigrating to the substrate 322 where they may be lost by recombination.When photons 370 illuminate the P-type layer 320, hole-electron pairsare generated in the P-type layer 320 and the holes migrate to the P+region 316 and the electrons migrate to the N+ region 318. This resultsin a photovoltaic potential and current can flow externally between theanode ohmic contact 314 and the cathode ohmic contact 334. Note that thehole-electron migration is in a lateral direction. These configurationsemploy a thin layer 320 for photon collection.

FIG. 4A depicts a cross section view of a multiple diode cell device 400fabricated by forming alternate N+ collecting junctions 418, 438, 458and P+ ohmic contact regions 416, 436, 456 to a lightly doped P-typeabsorbing layers 420, 440, 460 and employing a lateral collectionmethod. FIG. 4B depicts symbolic electrical representations 410, 430,450 of the device shown in FIG. 4A that comprises three series-connectedthermo-photo-voltaic diodes 410, 430, 450. Within each cell 410, 430,450 the diffusions alternate between N+ collection junctions 418, 438,458 and P+ ohmic contact regions 416, 436, 456 to the P-type layers 420,440, 460. The electron-hole pairs generated by photons absorbed by theP-type regions travel laterally to these N+ and P+ regions. Photonabsorption takes place in the thin lightly doped P-type layers 420, 440,460 that have been grown on a substrate 422 of un-doped wide bandmaterial such as semi-insulating gallium arsenide or silicon. The inband photon absorbing layers 420, 440, 460 may be N-type or P-typedepending on the semiconductor material used.

The N+ regions 418, 438, 458 and the P+ regions 416, 4436, 456 may beformed by any appropriate technique and may be minimized in area becauseof the high lifetime in the absorbing layer. Surface protection andisolation on the cut surfaces is provided by dielectric layer 412 thathas openings or vias to allow ohmic contact 414, 434, 454, 474 by adeposited metal film to the P+ region 416, 436, 456, and to the N+region 418, 438, 458.

The current flows laterally between the ohmic contacts and theinterconnecting metallization. Note there is no buried lateralconduction layer used in this structure.

Photon absorption occurs in the P-type layers 420, 440, 460 which may bedoped somewhat higher in the vicinity of the substrate 422 than theremainder of the P− regions 420, 440, 460 to form a diffusion barrier tokeep minority carriers from migrating into the substrate 422 where theymay be lost by recombination. When photons 470 illuminate the P-typelayers 420, 440, 460, hole-electron pairs are generated in the P-typelayer 420, 440, 460 and the holes migrate to the P+ regions 416, 436,456 and the electrons migrate to the N+ region 418, 438, 458. The unitcells 410, 430, 450 are defined by isolation trenches or cuts 424, 444that are etched into the substrate material between the first cell 410and second cell 430, and the second cell 430 and the third cell 450.These trenches or cuts 424, 444 may be made narrow and refilled topresent a smooth surface for ease of processing. Metallic interconnects434, 454 are made within the cuts 424, 444 between the cathode 418 ofthe first cell 410 and the anode 436 of the second cell, and between thecathode 438 of the second cell 430 and the anode 456 of the third cell450. When illuminated, this results in a photovoltaic potential betweenthe anode ohmic contact 414 of the first cell 410 and the cathode ohmiccontact 474 of the third cell 450, with the three cells being connectedin series. Note that the hole-electron migration is in a lateraldirection. These configurations employ a thin layer 420, 440, 460 forphoton absorption.

The thermophotovoltaic array is configured to be illuminated in the nearfield through the lower surface of the substrate 422. This provides thecapability of having a smooth flat surface that is ideal for forming anear-field gap with a photon-emitting surface. The dielectric orinsulating layers 412 and the metallization are selected for optimalback surface reflection, and the area of doped regions are minimized sothat the amount of reflection is maximized without degrading collectionefficiency.

FIG. 5A depicts an array of multiple diode devices 500 formed byalternating interdigitated N+ collection junctions 506, 510, 526, 530,546, 550, 566, 570, 586, 590 and P+ ohmic contact regions 504, 508, 512,524, 528, 532, 544, 548, 552, 564, 568, 572, 584, 588, 592 employing alateral collection method according to the present invention shown in aperspective view in FIG. 5A, in longitudinal sectional view A-A′ in FIG.5C and in a lateral sectional view B-B′ in FIG. 5D. FIG. 5B depictssymbolic electrical representations 518, 538, 558, 578, 598 of thedevice configuration shown in FIG. 5A that comprises fiveseries-connected photovoltaic diodes cells 518, 538, 558, 578, 598.

FIG. 5A through FIG. 5D depict similar diode configurations 518, 538,558, 578, 598 where each diode configuration includes N+ collectingjunctions and P+ ohmic contacts arranged in a parallel configuration.FIG. 5A through FIG. 5D illustrate a series connection of 5 photovoltaicdiodes with multiple contacts. Considering the first device 518 (and byanalogy, the second 538, third 558, fourth 578 and fifth 598 devices),the first device is fabricated by forming alternating a first P+ ohmiccontact region 504, a first N+ collection junction 506, a second P+ohmic contact region 508, a second N+ collection junction 510 and athird P+ ohmic contact region 512 to a lightly doped P-type absorbinglayer 514 employing a horizontal collection method. An anodemetallization 502 of the first diode 518 is connected to the first P+ohmic contact region 504, the second P+ ohmic contact region 508, andthe third P+ ohmic contact region 512. A cathode metallization 522 ofthe first diode 518, which also serves as an anode metallization to thesecond diode 538, is connected to the first N+ collection junction 506and the second N+ collection junction 510. Within each diode and betweeneach succeeding diode, the connections between diffusions alternatebetween N+ collection junctions 506, 510, 524, 528, 532, 546, 550, 564,568, 572, 586, 590 and P+ ohmic contact regions 504, 508, 512, 526, 530,544, 548, 552, 566, 570, 584, 588, 592 and to the collection regions ofthe P-type absorbing layers 514, 534, 554, 574, 594. The electron andholes generated by photons absorbed in the P-type regions 514, 534, 554,574, 594 travel laterally to the N+ collection junctions and P+ ohmiccontact regions, respectively. The absorbing layers are thin and aregrown on a substrate 520 of un-doped wide band gap material such assemi-insulating gallium arsenide or silicon. The absorbing layers may beP-type or N-type depending on the semiconductor material used.

The N+ collection junctions and the P+ ohmic contact regions may beformed by any appropriate technique and may be minimized in area becauseof the high lifetime in the absorbing layer particularly in the case ofgermanium as the absorbing material.

The surface protection and isolation in the trench areas is not shown inFIG. 5A for clarity. It is shown as layer 514, 516, or 612 in FIG. 5Cthrough FIG. 6F.

The dielectric layer has openings or vias to allow ohmic contact by adeposited metal film to the P+ ohmic contact regions and to the N+collection junctions.

Lateral conduction takes place by diffusion of carriers between theohmic contacts and along the interconnecting metal film. There is nolateral conduction layer required.

Photon absorption occurs in the P-type layers 514, 534, 554, 574, 594which may be doped somewhat higher in the vicinity of the substrate 520to form a diffusion barrier to keep minority carriers from migratinginto the substrate 520 where they may be lost by recombination. Whenphotons are absorbed in the P-type layers 514, 534, 554, 574, 594,hole-electron pairs are generated in the P-type layer 514, 534, 554,574, 594 and the holes migrate to the P+ ohmic contact regions and theelectrons migrate to the N+ collection junctions. The diodes are definedby isolation trenches 523, 543, 563, 583 that are etched into thesubstrate material between the first diode 518 and second diode 538,between the second diode 538 and third diode 558, between the thirddiode 558 and the fourth diode 578, and between the fourth diode and thefifth diode 598. These trenches 523, 543, 563, 583 may be made narrowand refilled to present a smooth surface for ease of processing.Metallic interconnects 522, 542, 562, 582 are made between the cathodeof the first diode 588 and the anode of the second diode 538, betweenthe cathode of the second diode 538 and the anode of the third diode538, between the cathode of the third diode 558 and the anode of thefourth diode 578, and between the cathode of the fourth diode 578 andthe anode of the fifth diode 598. When illuminated, this results in aphotovoltaic potential between the anode ohmic contact 502 of the firstdiode 518 and the cathode ohmic contact 580 of the fifth diode 598, withthe five diodes being connected in series to provide additive voltageoutput. Note that the hole-electron migration is in a lateral direction.

The thermophotovoltaic array is configured to be illuminated in the nearfield through in the lower surface of the substrate 520. This providesthe capability of having a smooth flat surface that is ideal for forminga near-field gap with a photon-emitting surface. The dielectric layer514 and the metallization 502, 522, 542, 562, 582, 580 are selected foroptimal back surface reflection for several reasons. It provides asecond pass to absorb in band photons and recycles below band photonsback to the emitter. The interdigitated junction regions may be formedby ion implantation, diffusion, or other appropriate techniques andalternate in a longitudinal and lateral direction.

FIG. 6A through FIG. 6F depict steps of a typical fabrication process600 for forming a multicell device according to the present invention.The example describes a germanium absorbing layer grown on a semiinsulating gallium arsenide substrate.

The description relies on the disclosed configuration shown in FIG. 4Aand described herein. These process steps may be easily expanded tofabricate smaller devices as well as larger scale arrays. FIG. 6Adepicts a thin layer of lightly doped P type germanium 680 epitaxiallygrown on a substrate 622 of undoped gallium arsenide. The germaniumlayer adjacent to the substrate 622 has a thin moderately doped P layerwhich forms a back surface reflector, a diffusion barrier which keepsminority carriers from diffusion into the substrate where they will belost due to recombination. FIG. 6B depicts P+ regions 616, 636, 656 andN+ regions 618, 638, 658 formed in the P− layer 680 by ion implantation.The P− layer regions between the P+ and N+ regions provide a lateralcollection region for electron-hole pairs generated by absorbingincident photons. FIG. 6C depicts trenches cuts 624, 644 through the Ptype germanium layer down to the substrate 622 providing isolationbetween the diodes formed by the P+ and N+ regions and the P typegermanium layer. FIG. 6D depicts a dielectric layer 612 deposited overthe surface. FIG. 6E depicts vias 684 that are cut to expose contactareas to the P+ and N+ regions. FIG. 6F depicts ohmic contacts 614, 634,654, 674 formed by metallization patterns for interconnection purposes.FIG. 6F represents the device shown in FIG. 4A.

The invention claimed is:
 1. A method for generating electrical energyusing a multicell photovoltaic monolithic semiconductor device,comprising: positioning a substrate common to all cells of the multicellphotovoltaic monolithic semiconductor device having a first surface forreceiving photon energy; forming an interface with a plurality oflateral collection regions on a second surface of the substrate oppositethe first surface; generating electron-hole pairs from absorbed photonenergy from the second surface by the plurality of lateral collectionregions; creating N+ collection junctions and P+ ohmic contact regionsin a surface of each of the plurality of lateral collection regions, theN+ collection junctions for collecting electrons and the P+ ohmiccontact regions for collecting holes from the electron-hole pairstraveling laterally in the plurality of lateral collection regions;providing isolation cuts between individual cells of the multicellphotovoltaic monolithic semiconductor device, the individual cellsformed by isolating adjacent P+ ohmic contact regions and N+ collectionjunctions; depositing dielectric layers on the surface of each of theplurality of lateral collection regions and on surfaces of the isolationcuts; and interconnecting N+ collection junctions and P+ ohmic contactregions between the individual cells in an array for forming themulticell photovoltaic monolithic semiconductor device.
 2. The method ofclaim 1, wherein each individual cell of the multicell photovoltaicmonolithic semiconductor device includes one or more diodeconfigurations, each diode configuration includes an N+ collectionjunction and a P+ ohmic contact region.
 3. The method of claim 1,wherein the first surface of the substrate is separated from an emittingheat source by less than one micron.
 4. The method of claim 1, furthercomprising recycling below band photons back to the substrate secondsurface by back surface reflection from the dielectric layers andinterconnections.
 5. The method of claim 1, wherein the dielectriclayers and interconnections comprise a back surface reflector forproviding a second pass for reflected in-band photons to be absorbed inthe plurality of lateral collection regions.
 6. The method of claim 1,further comprising fabricating and interconnecting multiple cells toform a monolithic large scale array.
 7. The method of claim 1, furthercomprising the incorporation of a thin layer of medium doped material inthe vicinity of the interface of the plurality of the lateral collectionregions and the second surface of the substrate for forming a diffusionbarrier to keep minority carriers from migrating into the substrate fromthe plurality of lateral collection regions where they would be lost byrecombination.
 8. The method of claim 1, further comprising providinglateral conduction by diffusion of carriers between ohmic contacts andalong interconnecting metallization layers, such that lateral conductionlayers imbedded in the body of the collecting regions are not required.9. The method of claim 1, further comprising creating electron-holepairs in the plurality of lateral collection regions by impingingphotons wherein electron-hole pairs created in the plurality ofcollection regions by impinging photons travel laterally to collectionjunctions and ohmic contact regions.
 10. The method of claim 1, furthercomprising the substrate first surface providing a smooth flat surfacefor forming a near-field gap with a photon energy-emitting surface. 11.The method of claim 1, wherein the plurality of lateral collectionregions are germanium semiconductor materials, wherein material bandgapsprovide a good match for converting energy from a low temperature heatsource.
 12. The method of claim 1, further comprising selecting thesubstrate from the group consisting of semi-insulating gallium arsenide,silicon, and indium phosphide.
 13. The method of claim 1, furthercomprising selecting the plurality of lateral collection regions fromthe group consisting of N-type and P-type material.
 14. The method ofclaim 1, further comprising selecting the plurality of lateralcollection regions from the group consisting of germanium, GaSb, GaInSb,GaInAs, GaInAsSb and InGaAs.
 15. The method of claim 1, wherein thephotovoltaic device is a thermophotovoltaic device.
 16. The method ofclaim 1, wherein the substrate first surface receives infrared andnear-infrared photon energy from an emitter heat source in a far field.17. A process for fabricating a multicell photovoltaic monolithicsemiconductor device, comprising the steps of: epitaxially growing alayer of lightly doped P-type germanium on a substrate of undopedgallium arsenide; forming P+ regions and N+ regions in the lightly dopedP-type germanium layer opposite the substrate by ion implantation;providing a lateral collection region in the lightly doped P-typegermanium layer between the P+ region and the N+ region forelectron-hole pair generation by absorbed incident photons; etchingisolation cuts through the P-type germanium layer down to the substratefor isolating diodes formed by the P+ region and N+ region and theP-type germanium layer; depositing a dielectric layer over the surfacecomprising the P+ region, the N+ region, the P-type layer and theisolation cuts; opening vias to expose contact areas to the P+ regionsand the N+ regions; and creating ohmic contacts by metallizationpatterns for interconnecting diodes and cells.
 18. The process of claim17, further comprising moderately doping the P-type layer adjacent thesubstrate to form a diffusion barrier to keep minority carriers fromdiffusing into the substrate.
 19. A method for generating electricalenergy using a multicell photovoltaic monolithic semiconductor device,comprising: positioning a substrate common to all cells of the multicellphotovoltaic monolithic semiconductor device having a first surface forreceiving photon energy; generating electron-hole pairs from absorbedphoton energy by a plurality of lateral collection regions adjacent to asecond surface of the substrate opposite the first surface; creating N+collection junctions and P+ ohmic contact regions in a surface of eachof the plurality of lateral collection regions; providing isolation cutsbetween individual cells of the multicell photovoltaic monolithicsemiconductor device; interconnecting N+ collection junctions and P+ohmic contact regions between the individual cells in an array forforming the multicell photovoltaic monolithic semiconductor device. 20.A method for generating electrical energy using a multicell photovoltaicdevice, comprising: providing a substrate common to all cells of themulticell photovoltaic device; forming a lateral collection region on asurface of the substrate; forming a plurality of P+ ohmic contactregions and an N+ ohmic contact regions in a surface of the lateralcollection region; connecting an anode contact to each P+ ohmic contactregion and connecting a cathode contact to each N+ ohmic contact region;providing electrical isolation cuts between individual cells of themulticell photovoltaic device; and generating a photovoltaic potentialbetween the anode contact and the cathode contact of each individualcell from the collection of photon energy by the lateral collectionregion.